Oxidation of sulfur dyes on cotton using hydrogen peroxide solutions



United States Patent 3,278,254 OXIDATION OF SULFUR DYES 0N COTTON USHNG HYDROGEN PEROXIDE SOLUTIONS Albert F. Chadwick, Wilmington, Deb, and Hugh 1!). Terhune, Greensboro, N.C., assignors to E. I. du Pont de Nemours and Company, Wilmington, Deb, a corporation of Delaware No Drawing. Filed June 5, 1964, Ser. No. 373,105

8 Claims. (Cl. 837) This application is a continuation-in-part of our applicatino Serial No. 331,930, filed December 19, 1963.

This invention relates to the oxidation of sulfur dyes and more particularly to their oxidation on cotton goods by means of aqueous hydrogen peroxide solutions.

Sulfur dyes are generally less expensive than vat dyes in many applications and they are widely used to dye cotton fabrics a wide variety of colors ranging from light green and blue to black. The dyeing operation generally involves impregnating a pre-scoured cotton fabric with a solution of the dye in its reduced form, steaming the impregnated fabric for a short time to increase penetration of the dye into the fibers, washing to remove excess dye then oxidizing the dye on the fabric to render it fast.

Oxidation of reduced sulfur dyes on the fabric is usually effected commercially by a short treatment in a sodium dichromate solution acidified with acetic acid. While such dichr-omate-acetic acid solutions produce dyed fabrics having good light and Wash fastness and wet and dry crock resistance, they nevertheless are unsatisfactory in several respects. Residual chrome salts present in the dyed fabric, even after thorough washing, give the fabric a harsh feel and cause severe friction with over-heating and burning of needles when the dyed fabric is handled on high speed sewing machines and the waste oxidizing solution creates a liquor disposal problem.

Aqueous hydrogen peroxide has not heretofore been regarded as an acceptable replacement for the conventional dichromate-acetic acid oxidizing solutions, since many of the sulfur dyes and notably the sulfur black dyes which are the most difiicult to oxidize, are dissolved from the fabric by the commonly used aqueous hydrogen peroxide solutions during the oxidation With the result that the color intensity of the treated fabric is far from satisfactory. It has now been discovered that sulfur dyes can be effectively and advantageously oxidized on cotton fabrics to give good color intensities with good light and wash fastness and good wet and dry crock resistance, using aqueous hydrogen peroxide solutions containing an agent which inhibits or prevents the loss of dye from the fabric to the peroxide solution.

It is an important object of the invention to provide an improved method of oxidizing sulfur dyes on cotton fabrics. A more specific object is the provision of such a method wherein a hydrogen peroxide solution is employed to oxidize the dyes while avoiding the abovementioned disadvantages attending the use of dichromateacetic acid solutions. Another object is the provision of such a method wherein the dye is oxidized using a peroxide solution containing an agent which inhibits or prevents transfer of the dye from the fabric to the solution. Still other objects will be apparent from the following description.

Prior commercial methods for dyeing cotton fabrics with sulfur dyes involve impregnating the fabric, preferably in pre-scoured condition, with an aqueous solution of the dye in its reduced form, steaming the impregnated fabric to insure penetration of the dye into the fibers of the fabric, washing the fabric to remove excess dye, then oxidizing the dye on the fabric by means of a sodium dichromate-acetic acid solution. The method of the invention differs from such prior methods in the kind of "ice solution employed in the oxidizing step wherein the dye is oxidized on the fabric.

In accordance with the invention, the fabric which has been impregnated with the reduced dye solution, is subjected to the oxidizing action of an aqueous hydrogen peroxide solution having a pH of 5 to about 9 and consisting essentially of (a) hydrogen peroxide and (b) a Water-soluble salt of magnesium, calcium, strontium or barium at concentrations, respectively, of 0.1 to 0.5 and 0.5 to 7%, based upon the weight of the solution.

Since the oxidation takes place rapidly, requiring only about 0.5 to 2 minutes at room temperature, it can be carried out readily in continuous operations wherein the dye-impregnated fabric in continuous length is passed continuously through the peroxide solution at a rate such as will provide the required residence time in the solution. Well-known devices for passing a fabric strand alternately over and under guide rolls positioned alternately above and submerged in a treating solution can be used effectively in carrying out such continuous operations. Although continuous operation is preferred, the oxidation can obviously be carried out batchwise, if desired.

The invention is based upon the unexpected discovery that the presence of a water-soluble salt of magnesium, calcium, strontium or barium in the peroxide solution inhibits or prevents transfer of the dye from the fabric to the solution during the oxidation treatment with the result that full colored dyed fabrics are obtained. To accomplish this result, the concentration of such salt in the per-oxide solution should be at least 0.5%, since concentrations substantially lower than about 0.5% are generally insufficiently effective to be practical. Concentrations ranging from about 0.5 to 7% are generally effective. Higher concentrations can be used but result in no added advantage to'compensate for the extra cost. The preferred concentrations range from about 1 to 5%, based upon the weight of the solution.

Any of the Water-soluble salts of the above-named metals can be used in practicing the invention, provided that the salt is inert towards or compatible with the hydrogen peroxide, e.g., does not catalyze decomposition of the peroxide or react to consume the peroxide, under the conditions of use. Examples of suitable salts are the acetates, chlorides and nitrates of magnesium, calcium, strontium and barium, and the sulfate of magnesium. Of these, the four nitrates and magnesium sulfate are preferred, and magnesium sulfate is especially preferred because of its over-all effectiveness and relative cheapness. The chloride salts, although effective, introduce chloride ions which may cause corrosion of metal equipment. Therefore, a metal salt other than a chloride will generally be used when the method of the invention is to be practiced in, for example, steel equipment. The permanganates, sulfites and iodides of the above metals are examples of salts which are not compatible with hydrogen peroxide under the conditions of use and, therefore, are unsuitable.

The concentration of hydrogen peroxide (H 0 in the oxidizing solution should be at least 0.1% for effective results while concentrations greater than about 0.5% should be avoided since at such greater concentrations the peroxide tends to attack the dye adversely. The preferred concentrations range from 0.15 to 0.4%, based upon the weight of the solution.

The peroxide solution employed should have a pH in the range of 5 to about 9. Effective oxidation is not realized under conditions more acidic than a pH of about 5 while under conditions more alkaline than a pH of about 9 it is not possible to hold the required amount of metal salt in solution. Preferably, the pH of the peroxide solution will range from about 6 to 8. Adjustment of the pH, if necessary, to within the effective range can be effected by the addition of appropriate amounts of any of the soluble acids or alkalies commonly employed for pH adjustment purposes, provided that use of materials such as sodium silicate which would tend to precipitate the metal cation of the salt should be avoided. Suitable acids are hydrochloric, nitric, sulfuric and acetic acids, the choice being dependent somewhat on the metal salt used. Suitable alkalies are the alkali metal hydroxides and ammonium hydroxide.

Oxidation of the dye with the peroxide solution is preferably effected at a temperature of about 60 to 140 F. However, lower and higher temperatures, e.g., up to 170 F. or higher, can he used but appear to offer no added advantage.

In the following examples, several yards of a 4-inch strip of a pre-scoured, unbleached, 3-yard cotton twill were freshly impregnated with a freshly prepared 10% solution of a commercial sulfur black dye in reduced form following the well-known conventional procedure. This involved impregnating the fabric at room temperature with about an equal weight of the reduced dye solution, steaming the impregnated fabric for about one minute, then squeezing it out to a liquor content about equal to the weight of the fabric. Such damp, dye-impregnated fabric was freshly prepared prior to each oxidation experiment. The commercial sulfur black dyes used were of the type known in the trade as Sulfur Black, Color Index No. 1, which are reaction products of 2,4- dinitrophenol and sodium polysulfide.

It was found that hydrogen peroxide solutions similar to those employed in the following examples, except that they contained no salt of the above-named metals, did not effectively oxidize the dye-impregnated fabric in that such solutions dissolved and transferred substantial amounts of the dye to the solutions with the result that in each case the treated fabric was off-shade (bronzy) or was of poor color intensity. The following examples show the effectiveness of the presence of such metal salts in the peroxide solution in overcoming such deficiencies, when such solutions are used under suitable conditions.

Example 1 A sample of dye-impregnated fabric freshly prepared as described above was passed through a peroxide oxidizing solution at a rate (about 1 yard per minute) to provide a residence time of 1 minute in the solution, then given a water rinse. The aqueous peroxide solution contained 0.18% H and 0.5% CaCl and had a pH of 3.5 (unadjusted). The color intensity of the resulting dyed fabric (oxidized) was considerably inferior to that of a corresponding sample of the dye-impregnated fabric which had been oxidized with -a conventional 1% sodium dichromate solution containing 1% acetic acid.

When the same peroxide solution was used after first adjusting its pH to 6.1 or 8.6, the color intensity and fastness of the treated fabric was essentially equal to that of fabric oxidized with the conventional dichromate-acetic acid solution.

Example 2 The procedure of Example 1 was essentially repeated using a peroxide solution having a pH of 7.5 and containing 0.18% H 0 and 2.0% CaCl The color intensity and fastness of the treated fabric was essentially equal to that obtained with the conventional dichromateacetic acid solution.

Similar results were obtained when the CaCl content was increased to either 3.0 or 5.0%.

Example 3 The procedure of Example 1 was repeated except that the peroxide solution used had a pH of 8.1 and contained, instead of calcium chloride, 1% magnesium sulfate (MgSO The color intensity of the treated fabric was essentially equal to, and the fabric absorbency was snperior to, that of fabric oxidized with the conventional dichromate-acetic acid solution.

Example 4 Example 3 was repeated except that the pH of the peroxide solution was 5.3. The color intensity of the treated fabric was essentially equal to, and the fabric absorbency was superior to, that of fabric oxidized with the conventional dichromate-acetic acid solution.

Example 5 Example 4 was repeated except that the peroxide solution contained 0.35% H 0 The color intensity of the treated fabric was essentially equal to that of fabric oxidized with the conventional dichromate-acetic acid solution.

Example 6 The procedure of Example 1 was repeated except that the peroxide solution used had a pH of 5.4 and contained instead of calcium chloride, 2% strontium nitrate (Sr(NO The treated fabric had good adsorbency and good color intensity, but a somewhat reddish case.

Example 7 The rocedure of Example 1 was repeated except that the peroxide solution used had a pH of 5.4 and contained, instead of calcium chloride, 2% BaCl -2H O. The treated fabric had good absorbency and good color intensity, but a somewhat reddish cast.

Example 8 A series of experiments similar to that of Example 3 were carried out using peroxide solutions containing magnesium sulfate (MgSO at concentrations of 0.049, 0.123, and 0.49%. The resulting treated fabric samples had poor color intensity and appeared gray when compared with fabric oxidized according to Example 3 or with the conventional dichromate-acetic acid solution.

Example 9 The procedure of Example 3 was repeated except that the peroxide solution contained 2% sodium sulfate (Na SO instead of magnesium sulfate. The treated fabric had very poor color intensity and a gray appearance.

In all of the foregoing examples, the fabric samples that were treated with peroxide solutions had a soft feel after simple rinsing, compared to samples treated with the conventional dichromate-acetic acid solution which had a distinctly harsh feel. Furthermore, the dyed samples that had been treated with peroxide solutions in accordance with the invention were of good color intensity, possessed good light and wash fastness and wet and dry crocking resistance, and compared favorably in these respects with dyed fabrics which had been oxidized with the conventional dichromate-acetic acid oxidizing solution. Such peroxide solutions extracted little if any dye from the fabric during their use.

Throughout the specification and claims, all concentrations expressed as percentage are by weight unless otherwise specified.

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:

1. In a process for dyeing a cotton fabric with a sulfur dye wherein the fabric is impregnated with a solution of the dye in its reduced form and the impregnated fabric is then treated with an oxidizing solution to effect oxidation of the dye on the fabric, the improvement comprising employing as said oxidizing solution an aqueous hydrogen peroxide solution having a pH in the range 5 to 9 and consisting essentially of hydrogen peroxide (H 0 at a concentration of 0.1 to 0.5% and a metal salt at a concentration of 0.5 to 7%, said salt being a water-soluble salt of a metal of the group consisting of magnesium, calcium, strontium and barium and compatible with hydrogen peroxide under the conditions of use.

2. In a process according to claim 1, wherein treatment of the impregnated fabric with the hydorgen peroxide solution is effected at a temperature of 60 to 140 F.

3. In a process according to claim 1, wherein the hydrogen peroxide solution employed has a pH in the range 6 to 8 and consists essentially of hydrogen peroxide (H at a concentration of 0.15 to 0.4% and said metal salt at a concentration of 1.0 to 5%.

4. Ina process according to claim 1, wherein the metal salt employed is a nitrate.

5. In a process according to claim 1, wherein the metal salt employed is a calcium salt.

6. In a process according to claim 1, wherein the metal salt employed is a magnesium salt.

7. In a process according to claim 1, wherein the metal salt employed is magnesium sulfate.

8. In a process according to claim 1, wherein the hydrogen peroxide solution employed contains 0.15 to 0.4% H 0 and 1 to 5% magnesium sulfate (MgSO References Cited by the Examiner UNITED STATES PATENTS 3,043,645 7/1962 Womble 8-37 FOREIGN PATENTS 110,367 3/1900 Germany. 552,735 4/1943 Great Britain.

J. HERBERT, Assistant Examiner. 

1. IN A PROCESS FOR DYEING A COTTON FABRIC WITH A SULFUR DYE WHEREIN THE FABRIC IS IMPREGNATED WITH A SOLUTION OF THE DYE IN ITS REDUCED FORM AND THE IMPREGANTED FABRIC IS THEN TREATED WITH AN OXIDIZING SOLUTION TO EFFECT OXIDATION OF THE DYE ON THE FABRIC, THE IMPROVEMENT COMPRISING EMPLOYING AS SAID OXIDIZING SOLUTION AN AQUEOUS HYDROGEN PEROXIDE SOLUTION HAVING A PH IN THE RANGE 5 TO 9 AND CONSISTING ESSENTIALLY OF HYDROGEN PEROXIDE (H2O2) AT A CONCENTRATION OF 0.5 TO 7%, SAID SALT BEING A WATER-SOLCONCENTRATION OF 0.5 TO 7%, SAID SALT BEING A WATER-SOLUBLE SALT OF A METAL OF THE GROUP CONSISTING OF MAGNESIUM, CALCIUM, STRONTIUM AND BARIUM AND COMPATIBLE WITH HYDROGEN PEROXIDE UNDER THE CONDITIONS OF USE. 